On November 10 the Air Force launched the last satellite in its secret Defense Support Program missile-warning constellation. Since 1970, the Defense Support Program has been the most important asset in the nation's entire military posture, because it is the only system that can reliably warn of a ballistic-missile attack against America no matter where that attack originates on the face of the earth.

Every strategy for averting nuclear war begins with being able to know whether the nation is under attack. If the military can't be sure of detecting an attack, then neither the threat of retaliation nor the possibility of active defense is credible.

So the fact that there are no more Defense Support Program satellites left to launch has provoked concern among policymakers, because the effort to develop a more capable successor that was begun in 1995 is running years behind schedule. Some analysts had hinted that if the Delta IV rocket launching the last satellite in the current constellation failed, the nation might face an imminent gap in missile warning. After all, satellites don't last forever, and some of the existing satellites have been in orbit for many years.

What if they stopped working before the next-generation system became operational? Could the nation be vulnerable to a surprise attack?

That possibility prompted a quick study of alternatives by the Pentagon. It turned out there aren't any alternatives that can take over the mission in a timely fashion, so there is no option but to press ahead with the planned replacement. That new system is called the Space Based Infrared System, or SBIRS ("sih-birs"), and like the legacy constellation it is designed to detect the heat generated by hostile missiles long before they reach their destination.

The baseline constellation of missile-warning satellites today and in the future will consist of at least four spacecraft carrying infrared telescopes in geosynchronous orbits plus two additional sensors piggybacked on electronic eavesdropping satellites in elliptical orbits.

The geosynchronous satellites occupy orbits 22,300 miles above the earth near the equator, from which position they can see nearly half of the planet while appearing to remain stationary relative to the surface (actually, they are turning at precisely the rate that the earth is rotating).

The sensors in elliptical orbit cover polar regions not easily visible to satellites above the equator -- regions from which enemy submarines might launch missiles. Because more than one satellite can see the areas of greatest concern (like Russia and China), it is possible to get multiple perspectives on the same event. But if you lose even one sensor from the baseline constellation, awareness begins to diminish. Lose two, and you could be blind to some threats.

The government won't say how many missile-warning satellites are currently operational. However, it appears that every Defense Support Program spacecraft launched over the last 20 years has greatly exceeded its projected service life. In some cases, the satellites have remained operational four times longer than expected. The implication is that with the launch of the most recent satellite, the military actually has much more missile-warning capability than is required to satisfy basic mission requirements.

That situation won't last indefinitely, so it really is important to push ahead with construction of the SBIRS constellation. But barring a series of very improbable developments, the military faces no near-term gap in its missile-warning capabilities, and still has the time to bring SBIRS to fruition with all of its originally planned performance features intact.